The objective of this project is the resolution of genetic and metabolic control mechanisms of mammalian hepatic phenylalanine hydroxylase. The proposed studies are centered around: (a) The immunological delineation of the number of isozymes of phenylalanine hydroxylase. (b) Hormonal, nutritional factors that have influence in maintaining a given steady state of phenylalanine hydroxylase activity in a minimal deviation hepatoma cell line in culture.